U.S. patent application number 12/525627 was filed with the patent office on 2010-04-15 for electrode catalyst for a fuel cell, and fuel cell using the same.
This patent application is currently assigned to NATIONAL INSTITUTE OF ADVANCED INDUSTRIAL SCIENCE. Invention is credited to Akari Hayashi, Kenichi Kimijima, Ichizou Yagi.
Application Number | 20100092830 12/525627 |
Document ID | / |
Family ID | 39674039 |
Filed Date | 2010-04-15 |
United States Patent
Application |
20100092830 |
Kind Code |
A1 |
Hayashi; Akari ; et
al. |
April 15, 2010 |
ELECTRODE CATALYST FOR A FUEL CELL, AND FUEL CELL USING THE
SAME
Abstract
To provide an electrode catalyst for fuel cells, which is
obtained efficiently by a simple process, without using a silica
template, unlike in the conventional process, which is relatively
large in mesopore diameter (5 nm or more), which enables catalyst
particles deposited stably in the mesopores, and which can readily
develop a highly triple-phase interface state. Such an electrode
catalyst for a fuel cell is constituted with: a mesoporous carbon
support obtained by heating and baking for carbonization a mixture
of a surfactant and carbon precursors; and catalyst particles
carried by the support. Further, it is possible to obtain a fuel
cell, which has: a fuel electrode; an air electrode; and an
electrolyte membrane interposed between the electrodes, in which at
least one of the fuel electrode and the air electrode contains the
electrode catalyst.
Inventors: |
Hayashi; Akari; (Tokyo,
JP) ; Yagi; Ichizou; (Tokyo, JP) ; Kimijima;
Kenichi; (Tokyo, JP) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Assignee: |
NATIONAL INSTITUTE OF ADVANCED
INDUSTRIAL SCIENCE
Tokyo
JP
|
Family ID: |
39674039 |
Appl. No.: |
12/525627 |
Filed: |
January 30, 2008 |
PCT Filed: |
January 30, 2008 |
PCT NO: |
PCT/JP2008/051406 |
371 Date: |
August 3, 2009 |
Current U.S.
Class: |
429/481 ;
502/180 |
Current CPC
Class: |
H01M 4/92 20130101; H01M
4/926 20130101; H01M 2008/1095 20130101; Y02E 60/50 20130101; H01M
4/9083 20130101; H01M 4/90 20130101 |
Class at
Publication: |
429/30 ;
502/180 |
International
Class: |
H01M 4/96 20060101
H01M004/96; B01J 21/18 20060101 B01J021/18 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 1, 2007 |
JP |
2007-023252 |
Claims
1. An electrode catalyst for a fuel cell, comprising: a mesoporous
carbon support obtained by heating and baking for carbonization a
mixture of a surfactant and carbon precursors; and catalyst
particles carried by the support.
2. A fuel cell, having: a fuel electrode; an air electrode; and an
electrolyte membrane interposed between the electrodes, wherein at
least one of the fuel electrode and the air electrode contains the
electrode catalyst according to claim 1.
Description
TECHNICAL FIELD
[0001] The present invention relates to an electrode catalyst for a
fuel cell, and to a fuel cell using the same.
BACKGROUND ART
[0002] Fuel cells, especially polymer electrolyte fuel cells
(PEFCs), have recently attracted remarkable attention as energy
sources of transportation means, from the viewpoint that they work
at low temperatures and can be formed in small-sizes.
[0003] PEFCs generally have a structure in which a fuel electrode
(anode) and an air electrode (cathode) are disposed so as to
sandwich a solid polymer electrolyte membrane between those, and in
which a fuel, such as hydrogen, natural gas, or methanol, is
supplied to the anode and an oxidizing agent, such as oxygen, or
the air, is supplied to the cathode. Hydrogen ions and electrons
are generated by an oxidation reaction on the anode. The hydrogen
ions are transferred to the cathode by the solid polymer
electrolyte membrane, and the transferred hydrogen ions are allowed
to react with the oxidizing agent supplied from the outside, to
generate water. On the anode, on the other hand, the produced
electrons reach the cathode through any external circuits, upon
which current flows in that structure.
[0004] As one measures taken to improve the power generation
characteristics of such PEFCs, it is important to enhance the
activity of catalysts to be used in the anode and cathode.
Developments and studies of the catalysts themselves and catalyst
supports which carry the catalysts thereon, have been
enthusiastically made.
[0005] In particular, in order to run the catalyst reaction
efficiently on each electrode of those anode and cathode, it is
necessary to positively create the portion where three phases of
gas (oxygen and hydrogen which are active materials), ionomers as
the electrolyte, and the catalyst are united in each electrode,
that is, a triple-phase interface. Thus, various catalyst supports
have been proposed which are to increase that triple-phase
interface as much as possible.
[0006] As one of typical catalyst supports, there are proposed
amorphous microporous carbon powders made, for example of,
activated carbon and carbon black (Non-patent Document 1).
[0007] This activated carbon generally has a primary particle
diameter of 40 to 50 nm, and is provided with micropores (2 nm or
less) in a particle. Catalyst metal particles are carried in the
pores of this support or on the surface of the support, and are
mixed with Nafion ionomers. Under this state, a catalyst layer is
formed. When a fuel cell catalyst layer is actually formed, the
catalyst particles and ionomers are present in macropores formed
between carbon black particles, and oxygen gas passed through pores
and protons transported by the ionomers are allowed to react with
each other. This shows that it is fairly reasonable to say that the
catalyst metal particles exist at the triple-phase interface.
[0008] However, when a conventional carbon black support is used,
there arises the problem that catalyst particles carried in
micropores cannot be brought into contact with ionomers and do not
contribute to the reaction, resulting in a low catalyst utilization
(efficiency). Further, the stability of the catalyst particles
carried on this support is not so high. Thus, if the catalyst is
mixed with the ionomers required during the course of assembly of
an actual fuel cell, there arises the problem that the catalyst
particles slip out of the carbon black support, and it is also
pointed out that a platinum catalyst and a carbon support are
deteriorated in a long-term operation.
[0009] In order to solve these problems, there is proposed "a
catalyst support for fuel cells, which is obtained by combining
mesoporous carbon particles with carbon materials, such as carbon
black" (Patent Document 1).
[0010] This mesoporous carbon particles are those described as
"almost no pore having a size of 10 nm or more exists, the
distribution of the pores is concentrated at 2 to 3 nm, and pores
are present in the whole range of the measured pore diameter
distribution".
[0011] However, in order to obtain these mesoporous carbon
particles, it is necessary to adopt the following complicated
steps. Specifically, porous particles, such as silica (silica
template) and titania, which have an intended pore distribution
(mesoporous), are made to adsorb and/or be impregnated with a
carbon-containing molecule, such as sucrose. Then, the resultant
molecule is carbonized in an inert atmosphere, and then the
template particles, such as silica, are dissolved and/or removed by
hydrofluoric acid, NaOH/EtOH, or the like.
[0012] Further, in this case, mesoporous silica is in such a state
that rod-like carbon is formed in a pore portion of the mesoporous
silica, and that these carbon rods are bonded by carbon produced by
carbonization in micropores of the mesoporous silica. Thus, the
portion, called mesopores, of the mesoporous carbon material
constitutes the wall of the mesoporous silica, and only relatively
small mesopores of pore size about 3 nm or less are obtained.
Therefore, this mesoporous carbon synthesized through silica is
unnecessarily sufficient as the support that develops the above
triple-phase interface.
[0013] Further, recently, there is proposed "an electrode catalyst
for fuel cells, which uses a mesoporous carbon material as a
support, in which the carbon material has a surface resistance of
250 m.OMEGA./cm.sup.2 or less measured under a pressure of 75.4
kgf/cm.sup.2 and the average diameter of the mesopores is 2 to 20
nm" (Patent Document 2).
[0014] However, in order to obtain this mesoporous carbon material,
it is necessary to combine the steps of: (a) a step of producing
100 parts by mass of a precursor mixture including 5 to 15 parts by
mass of phenanthrene, 10 to 35 parts by mass of an acid, and 55 to
80 parts by mass of a solvent; (b) a step of impregnating the
mesoporous silica with the above precursor mixture; (c) a step of
heat-treating the resultant product obtained in the above step (b);
(d) a step of carbonizing the resultant product obtained in the
above step (c); and (e) a step of removing the above mesoporous
silica, by using a solvent capable of selectively dissolving only
silica. Thus, this mesoporous carbon material has the disadvantage
that the processes are quite complicated, similar to that in the
above Patent Document 1.
[0015] A method is also proposed in which a solution including a
surfactant and carbon precursors is applied to a silicon substrate,
and then the coated membrane is dried, baked and carbonized, to
form a carbon membrane having mesopores on the substrate
(Non-patent Document 2).
[0016] However, this reference relates to a method of forming a
non-self-standing carbon membrane having mesopores, and nothing is
described as to the method preparing self-standing mesoporous
carbon fine-particles, and also it is needless to say that nothing
is taught in relation to the uses and applications of the carbon
fine-particles. [0017] Non-patent Document 1: edited by Konosuke
IKEDA, "EVERYTHING IN FUEL CELLS", p. 138-139, Nippon Jitsugyo
Publishing [0018] Non-patent Document 2: Chem. Commun. 2005, p.
2125-2127 [0019] Patent Document 1: JP-A-2004-71253 ("JP-A" means
unexamined published Japanese patent application) [0020] Patent
Document 2: JP-A-2006-321712
SUMMARY OF THE INVENTION
Problems that the Invention is to Solve
[0021] It is an object of the present invention to provide an
electrode catalyst for fuel cells, which is obtained efficiently by
a simple process, without using a silica template, unlike in the
conventional process, which is relatively large in mesopore
diameter (5 nm or more), which enables catalyst particles to be
carried stably in the mesopores, and which can readily develop a
highly triple-phase interface state. It is another object of the
present invention to provide a fuel cell using that electrode
catalyst.
Means to Solve the Problems
[0022] The inventors of the present invention, having made earnest
studies as to the above problems in the conventional techniques,
have found that when using, as a support, a mesoporous carbon
material that is prepared by a specified method, the above problems
can be solved, thereby to attain the present invention.
[0023] That is, according to this application, the following
inventions are provided.
[0024] (1) An electrode catalyst for a fuel cell, comprising: a
mesoporous carbon support obtained by drying, baking, and
carbonizing a mixture of a surfactant and carbon precursors; and
catalyst particles carried by the support.
[0025] (2) A fuel cell, having: a fuel electrode; an air electrode;
and an electrolyte membrane interposed between the electrodes,
wherein at least one of the fuel electrode and the air electrode
contains the electrode catalyst according to the above (1).
ADVANTAGEOUS EFFECTS OF THE INVENTION
[0026] (1) The electrode catalyst of the present invention for fuel
cells is specified by the use of, as a support, a mesoporous carbon
support which is obtained by drying, baking, and carbonizing a
mixture of a surfactant and carbon precursors. A mesoporous carbon
material obtained in the above, can be synthesized by such a simple
step that the surfactant and the carbon precursors are baked and
carbonized, by utilizing self-organization of those, thereby to
conduct finally the carbonization and the removal of the surfactant
at the same time.
[0027] (2) The mesoporous carbon particles obtained by the
synthetic method as in the above, have the physical properties of:
a specific surface area of 500 to 700 cm.sup.2/g; a regular
mesoporous structure; and an average diameter of 5 to 10 nm.
[0028] (3) A conventional mesoporous carbon material that has been
used as this type of catalyst support, needs to adopt the following
complicated steps of: making to adsorb and impregnated porous
particles, such as silica (silica template) and titania, having an
intended pore distribution (mesoporous), with a carbon-containing
molecule, such as sucrose; carbonizing the carbon-containing
molecule in an inert atmosphere; and then dissolving/removing the
template particles, such as silica, by hydrofluoric acid,
NaOH/EtOH, or the like. However, the mesoporous carbon material
according to the present invention, has the advantage that a highly
active catalyst support can be obtained by a simple process,
without adopting such a complicated process.
[0029] (4) Further, the above conventional catalyst support is in
such a state that rod-like carbons are formed in pore portions of
mesoporous silica and these carbon rods are bonded by carbon
produced by carbonization in micropores of the mesoporous silica.
Thus, the portion, called mesopore, of the mesoporous carbon
material constitutes the wall of the mesoporous silica, and only
relatively small mesopores are obtained. Thus, this mesoporous
carbon synthesized through silica is unnecessarily sufficient as
the support that develops the triple-phase interface, which is an
important factor to determine improvement in a catalyst
activity.
[0030] Contrary to the above, since the catalyst support according
to the present invention has the aforementioned specific physical
properties, the catalyst particles can be stably carried in a large
amount in mesopores, and also the contact between polymer
electrolyte ionomer and the catalyst particles in the mesopores can
be enhanced, thereby to make it possible to attain the triple-phase
interface state with ease.
[0031] (5) Further, the fuel cell according to the present
invention is more resistant to deterioration of the support than a
conventionally known fuel cell using carbon black as a support.
Further, even if platinum particles are dissolved in a solution,
they are trapped in pores, so that the loss of platinum is reduced,
and thus the fuel cell of the present invention is high in the
activity and excellent in the durability.
[0032] Further, even if H.sub.2O.sub.2 which is a cause of
deteriorations is occurred in pores, it can be reduced to H.sub.2O
by another catalyst particles existing in the pores, and only a
limited amount of ionomers can exist in the pores. Thus, the growth
of the catalyst particles can be restricted, leading to prolonged
catalyst life, and making it possible to operate the fuel cell of
the present invention in a long term.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIG. 1 A nitrogen adsorption isotherm and a pore
distribution curve of the mesoporous carbon material obtained in
Synthetic Example 1
[0034] FIG. 2 A SEM photograph of the mesoporous carbon material
obtained in Synthetic Example 1
[0035] FIG. 3 A TEM photograph of the mesoporous carbon material
obtained in Synthetic Example 1
[0036] FIG. 4 An X-ray structural diffraction diagram of the
mesoporous carbon material obtained in Synthetic Example 1
[0037] FIG. 5 A nitrogen adsorption isotherm, and a curve showing
the relationship between the amount of Pt deposited and the pore
distribution of the mesoporous carbon material, with respect to the
Pt deposited electrode catalyst obtained in Example 1
[0038] FIG. 6 A SEM photograph of the 30% Pt deposited electrode
catalyst
[0039] FIG. 7
[0040] A TEM photograph of the 30% Pt deposited electrode catalyst
(at a low magnification)
[0041] FIG. 8 A TEM photograph of the 30% Pt deposited electrode
catalyst (at a high magnification)
[0042] FIG. 9 An oxygen reduction reactive curve (sweeping in the
direction of negative potential) of the cathodes of Example 1
(20-wt % Pt deposited catalyst) and Reference Example
[0043] FIG. 10 An oxygen reduction reactive curve (when no Nafion
is present and after Nafion is cast: sweeping in the direction of
positive potential) of the cathode of Example 1 (30-wt % Pt
deposited catalyst)
[0044] FIG. 11 An oxygen reduction reactive curve (dependency on
Nafion-diluting solvents: sweeping in the direction of positive
potential) of the cathode of Example 1 (30-wt % Pt deposited
catalyst)
[0045] FIG. 12 An oxygen reduction reactive curve (sweeping in the
direction of negative potential) of the cathode of Example 2
[0046] FIG. 13 An oxygen reduction reactive curve (sweeping in the
direction of negative potential) of the cathodes of Example 1 and
Comparative Example 1
[0047] FIG. 14 A cyclic voltammogram of the cathode of Example 1 in
an argon atmosphere
[0048] FIG. 15 A cyclic voltammogram of the cathode of Comparative
Example 2 in an argon atmosphere
[0049] FIG. 16 An oxygen reduction reactive curve (sweeping in the
direction of negative potential) of the cathode of Example 1
[0050] FIG. 17 An oxygen reduction reactive curve (sweeping in the
direction of negative potential) of the cathode of Comparative
Example 2
[0051] FIG. 18 A nitrogen adsorption isotherm, and a curve showing
the relationship between the amount of Pt deposited and the pore
distribution of the mesoporous carbon material, with respect to
Example 3
[0052] FIG. 19 A SEM photograph of the 30% Pt-ac electrode
catalyst
[0053] FIG. 20 A TEM photograph of the 30% Pt-ac electrode catalyst
(at a low magnification)
[0054] FIG. 21 A TEM photograph of the 30% Pt-ac electrode catalyst
(at a high magnification)
[0055] FIG. 22 An oxygen reduction reactive curve (sweeping in the
direction of positive potential) of the electrode catalyst (30-wt %
Pt-ac electrode catalyst) of Example 3 (also shown the oxygen
reduction reactive curve of the electrode catalyst (30-wt % Pt
deposited electrode catalyst) of Example 1)
[0056] FIG. 23 A SEM photograph of the 30% Pt--Cl electrode
catalyst
[0057] FIG. 24 A TEM photograph of the 30% Pt--Cl electrode
catalyst
[0058] FIG. 25 A TEM photograph of the 30% Pt--Cl electrode
catalyst
[0059] FIG. 26 An oxygen reduction reactive curve: sweeping in the
direction of positive potential) of the electrode catalyst (30-wt %
Pt--Cl electrode catalyst) of Example 4 (also shown the oxygen
reduction reactive curves of the electrode catalyst (30-wt % Pt
deposited electrode catalyst) of Example 1 and the electrode
catalyst (30-wt % Pt-ac electrode catalyst) of Example 3)
BEST MODE FOR CARRYING OUT THE INVENTION
[0060] The electrode catalyst of the present invention for fuel
cells is characterized by the use of, as a support, a mesoporous
carbon support obtained by baking and carbonizing a mixture of a
surfactant and carbon precursors.
[0061] This mesoporous carbon material can be synthesized by such a
simple process in which a surfactant and carbon precursors are
baked and carbonized by making use of self-organization of those,
to carry out finally the carbonization of the carbon precursors and
the removal of the surfactant at the same time.
[0062] Mesoporous carbon particles obtained by the above synthetic
method have the following physical properties: specific surface
area of 500 to 700 cm.sup.2/g, regular mesoporous structure, and
average particle diameter of 5 to 10 nm.
[0063] Next, the method of synthesizing the mesoporous carbon
material to be used in the present invention will be explained in
detail.
[0064] The synthetic method in the present invention utilizes the
self-organization of a surfactant and carbon precursors, without
using a silica template.
[0065] Though no particular limitation is imposed on the
surfactant, a nonionic surfactant is preferably used as the
surfactant, from the viewpoint of its effective action on the
self-organization with the carbon precursor.
[0066] As the nonionic surfactant, use may be made, for example, of
any of tri-block copolymers which have various polymerization
ratios and a molecular weight of about 2,000 to about 13,000, and
which are consisted of polyethylene oxide-polypropylene
oxide-polyethylene oxide (PEO-PPO-PEO) or polypropylene
oxide-polyethylene oxide-polypropylene oxide (PPO-PEO-PPO); as well
as polyoxyethylene alkyl ethers in which the alkyl group has 12 to
18 carbon atoms, polyoxyethylene octyl phenyl ether,
polyoxyethylene nonyl phenyl ether, sorbitan monopalmitate,
sorbitan monolaurate, sorbitan monostearate, sorbitan distearate,
sorbitan monooleate, sorbitan sesquioleate, sorbitan trioleate,
polyoxyethylenesorbitan monolaurate, polyoxyethylenesorbitan
monopalmitate, polyoxyethylenesorbitan monostearate,
polyoxyethylenesorbitan monooleate, polyethylene glycol
monolaurate, polyethylene glycol monostearate, polyethylene glycol
distearate, polyethylene glycol monooleate, oleic acid
monoglyceride, stearic acid monoglyceride,
polyoxyethylenelaurylamine, and polyoxyethylenestearylamine.
Particularly, the tri-block copolymer or the polyoxyethylene alkyl
ether is preferably used in the reaction system in the present
invention.
[0067] Any material may be used as the carbon precursor without any
particular limitation as long as it is a carbon-containing
compound, is polymerized in the presence of a surfactant, and is
made into a mesoporous carbon material after baking and carbonizing
the polymerized product. It is preferable to use, as the carbon
precursor, an organic compound with a benzene ring having an OH
group thereon, in combination with an organic compound having a CO
group, from the viewpoint of undergoing polymerization readily.
[0068] Examples of the organic compound with a benzene ring having
an OH group thereon, include phenols, such as phenol and
resorcinol; and examples of the organic compound having a CO group
include aldehydes, such as formaldehyde and acetaldehyde.
[0069] The carbon precursor preferably used in the present
invention is resorcinol and formaldehyde.
[0070] There is no particular limitation to the ratio of the carbon
precursor to the surfactant to be used. When resorcinol and
formaldehyde are used as the carbon precursor, this ratio by weight
is preferably set, for example, to:
resorcinol:formaldehyde:surfactant=1:0.31:0.57, in terms of weight
ratio.
[0071] In the present invention, a mesoporous carbon material may
be fundamentally obtained by baking and carbonizing a mixture of
the above surfactant and the above carbon precursor. It is
preferable to adopt an embodiment in which a mixture of the
surfactant and the carbon precursor is dissolved in an organic
solvent/water, followed by heating preferably at 95 to 105.degree.
C., to thereby first polymerize the carbon precursor. In that case,
as the organic solvent, any material may be used without any
particular limitation insofar as it dissolves the reaction
materials or is miscible with water and, for example, an alcohol is
used. The organic solvent preferably used in the present invention
is ethanol.
[0072] Further, it is preferable to use a reaction accelerator,
such as triethyl orthoacetate, or a reaction aid, to accelerate a
carbonization reaction between the surfactant and the carbon
precursor.
[0073] Further, the above reaction may be run under any of acidic
condition and alkaline condition.
[0074] In the case of under an acidic condition, a mesoporous
carbon material like a large bulk is obtained, alternatively in the
case of under an alkaline condition, a mesoporous carbon material
like a fine powder is obtained.
[0075] In the present invention, after the above step of
polymerizing the carbon precursor, the polymerized carbon precursor
is baked preferably in an inert gas atmosphere, such as argon or
nitrogen, to carbonize the polymer and also to remove the
surfactant at the same time, thereby obtaining the target
mesoporous carbon material.
[0076] The baking and carbonization conditions can be appropriately
determined, in consideration of the types of surfactant and carbon
precursor to be used and the physical properties of a desired
carbon porous material.
[0077] In the present invention, for example, the baking and
carbonization conditions can be set to: at 400.degree. C. for 3
hours, and then at 800.degree. C. for 6 hours.
[0078] There is no particular limitation to the catalyst to be
carried (deposited) by the above specific mesoporous carbon
material according to the present invention, and use may be made of
any of conventionally known metal catalysts, and the like.
[0079] Examples of metals used for these metal catalysts may
include the VIII-group elements, such as platinum, ruthenium,
palladium, osmium, iridium, rhodium, Fe, Co, and Ni; the I-group
elements, such as gold, silver, and Cu; the IV-group elements, such
as Ti and Zr; the V-group elements, such as V and Nb; and the
VI-group elements, such as Cr, Mo, and W. It is preferable to use
platinum among these metals.
[0080] These metals may be used either singly or in combinations of
two or more. The metal catalyst is preferably in the form of
particles, and more preferably in the form of metal particles
having an average particle diameter of about 1 to about 5 nm.
[0081] Although no particular limitation is imposed on the ratio by
mass of the metal catalyst to the support, the ratio of the metal
catalyst is preferably 10 to 60% by mass, with respect to the
support.
[0082] As the method of supporting the metal catalyst on the
mesoporous carbon support, any of conventionally known support
methods may be adopted. For example, use may be made of a method in
which a catalyst metal precursor solution is dipped (impregnated),
and then the metal catalyst precursor is reduced; or a method in
which the support is impregnated with the metal catalyst colloid
solution.
[0083] In order to specifically obtain a support catalyst by the
impregnation method, for example, the following method can be
taken. That is, an aqueous solution including the metal catalyst
precursor, such as tetraammineplatinate (II) nitrate, is added to
the mesoporous carbon powder, and the resultant solution is
stirred. The solution is then dried at ambient temperature under a
nitrogen atmosphere for 1 hour under stirring, and then the solvent
is completely vaporized at 80.degree. C. to dry. Then, a solvent,
such as dichloromethane, is added thereto, and the solution is
stirred, followed by distilling off the solvent, thereby making the
platinum catalyst precursor adsorb to the inside of hydrophobic
pores. Then the adsorbed precursor is heated in an inert
atmosphere, to reduce the same.
[0084] The main point for the electrode catalyst of the present
invention for fuel cells is the use of, as a support, a mesoporous
carbon support that is obtained by drying, baking, and carbonizing
a mixture of a surfactant and carbon precursors. The thus-obtained
mesoporous carbon material can be synthesized by such a simple
process in which the surfactant and the carbon precursors are baked
and carbonized by making use of self-organization of those, to
finally carry out the carbonization and the removal of the
surfactant at the same time.
[0085] Mesoporous carbon material particles obtained by the above
synthetic method have the physical properties: specific surface
area of 500 to 700 cm.sup.2/g, regular mesoporous structure, and
average particle diameter of 5 to 10 nm.
[0086] As mentioned above, the conventional mesoporous carbon
material that has been used as this type of catalyst support, needs
to adopt the following complicated steps: i.e. porous particles,
such as silica (silica template) and titania, having an intended
pore distribution (mesoporous) are made to adsorb and be
impregnated with a carbon-containing molecule, such as sucrose, and
the carbon-containing molecule is carbonized in an inert
atmosphere, and then the template particles, such as silica, are
dissolved/removed by hydrofluoric acid, NaOH/EtOH, or the like.
However, the mesoporous carbon material according to the invention
has the advantage that a highly active catalyst support can be
obtained by a simple process, without adopting such a complicated
process.
[0087] Further, the above conventional catalyst support is in such
a state that rod-like carbon is formed in a pore portion of
mesoporous silica, and these carbon rods are bonded by carbon
produced by carbonization in micropores of the mesoporous silica.
Thus, the portion, called mesopore, of the mesoporous carbon
material constitutes the wall of the mesoporous silica, and only
relatively small mesopores are obtained. Therefore, this mesoporous
carbon synthesized through silica is unnecessarily sufficient as
the support that develops the triple-phase interface, which is an
important factor to determine improvement in the above catalyst
activity.
[0088] Contrary, since the catalyst support according to the
present invention has the aforementioned specific physical
properties, the catalyst particles can be stably supported in a
large amount in mesopores, and also the contact between polymer
electrolyte ionomer and catalyst particles in the mesopores can be
enhanced, thereby to make it possible to attain the triple-phase
interface state readily.
[0089] As the polymer electrolyte ionomer, though any
conventionally known one may be used, it is preferable to use
Nafion. As the diluting solvent for Nafion, a solvent, for example,
an alcohol-type solvent, such as ethanol, 1-propanol or 2-propanol,
or a water/alcohol-type solvent, such as water/ethanol, may be
used, and it is preferable to use a Nafion solution diluted with a
more hydrophobic solvent, such as 2-propanol, to make it possible
to add much Nafion in the pores to thereby accelerate the formation
of a triple-phase interface, because the inside of the pores of the
mesoporous carbon material is hydrophobic.
[0090] The fuel cell according to the present invention is
characterized that it is provided with a fuel electrode, an air
electrode, and an electrolyte membrane interposed between those
electrodes, in which at least one of the fuel electrode and the air
electrode contains the aforementioned specific electrode
catalyst.
[0091] This fuel cell is, though not particularly limited to,
preferably a polymer electrolyte fuel cell (PEFC).
[0092] The PEFC according to the present invention may have any
structure insofar as it includes a solid electrolyte membrane, the
fuel electrode and the air electrode, and at least one of the fuel
electrode and the air electrode contains the above electrode
catalyst. For example, the PEFC includes: a membrane-electrode
assembly (MEA), which contains the above fuel cell electrode
catalyst and a gas diffusion layer (GDL) which holds the solid
electrolyte membrane and is formed of a carbon powder, and the
like; and a product obtained by sandwiching the MEA between
separators.
[0093] For example, the membrane-electrode assembly (MEA) may be
produced, by casting, a dispersion solution of the above support
catalyst on a carbon rod, followed by drying, and further by
casting, a polymer electrolyte solution containing Nafion and the
like, followed by drying.
[0094] Further, when the electrode catalyst according to the
present invention is applied to a gas diffusion layer (GDL), it can
be used in the same structure as in the case of a conventional fuel
cell catalyst layer. In this case, a process of mixing an ionomer
with the catalyst is required. Even if the electrode catalyst of
the present invention for fuel cells is mixed with the ionomer, the
catalyst particles can be hold stably inside of the mesopores.
[0095] Further, the fuel cell according to the present invention is
more resistant to deterioration of the support than a
conventionally known fuel cell using carbon black as the support.
Further, even if platinum particles are dissolved in the solution,
they are trapped in pores, leading to a reduction in the loss of
platinum, and thus the fuel cell according to the present invention
is high in the activity and is excellent in the durability.
[0096] Further, even if H.sub.2O.sub.2 which is a cause of the
deteriorations is occurred in pores, it can be reduced to H.sub.2O
by another catalyst particles existing in the pores, and also only
a limited amount of ionomers can exist in the pores. Thus, the
growth of the catalyst particles can be restricted, leading to a
prolonged catalyst life, enabling a long-term operation.
EXAMPLES
[0097] The present invention will be described in more detail based
on the following examples.
Example 1
Synthetic Example 1 of a Mesoporous Carbon Material
Acidic Condition
[0098] Resorcinol (manufactured by Wako Pure Chemical Industries
Ltd.) in an amount of 1.65 g was dissolved in 4.35 g of deionized
water/5.75 g of ethanol (manufactured by Wako Pure Chemical
Industries Ltd.)/0.15 mL of 5M hydrochloric acid (manufactured by
Wako Pure Chemical Industries Ltd.). To the resultant solution,
0.945 g of Pluronic F-127 (manufactured by SIGMA) was added, to
dissolve the resultant mixture completely. Then, 1.2 g of triethyl
orthoacetate (manufactured by Wako Pure Chemical Industries Ltd.)
and 1.35 g of 37% formaldehyde manufactured by Wako Pure Chemical
Industries Ltd.) were added thereto, and the resultant mixture was
stirred at 30.degree. C. for 20 minutes. This solution was poured
into a Teflon (registered trademark) container, which was then
heated at 105.degree. C. for 6 hours in an air blowing
thermohygrostat (DMK300, manufactured by Yamato Scientific Co.,
Ltd.), to polymerize resorcinol and formaldehyde.
[0099] The thus-obtained orange solid was poured onto a quartz
boat, followed by heating over three days according to a day-by-day
condition in a nitrogen atmosphere, with an infrared lamp heating
treatment device (ULVAC-RIKO, SSA-P610C).
[0100] First day: the temperature of the system was raised at a
rate of 1.degree. C. per minute from 20.degree. C. to 400.degree.
C. Then, the temperature was kept at 400.degree. C. for 3
hours.
[0101] Second day and third day: the temperature of the system was
raised at a rate of 2.degree. C. per minute from 20.degree. C. to
800.degree. C. Then, the temperature was kept at 800.degree. C. for
3 hours.
[0102] The thus-obtained mesoporous carbon material was produced in
an amount of 0.8 g as a black block (bulk).
Synthetic Example 2 of a Mesoporous Carbon Material
Alkaline Condition
[0103] The reaction was run in the same manner as in Synthetic
Example 1, except that 1.0 mL of 0.5M NaOH (manufactured by Wako
Pure Chemical Industries Ltd.) was used in place of hydrochloric
acid in Synthetic Example 1. As a result, 0.8 g of a mesoporous
carbon material was obtained as a black powder.
(Specific Surface Area of the Mesoporous Carbon Material Obtained
in Synthetic Example 1 and Average Diameter of Mesopores)
[0104] The measurement of adsorption of nitrogen to the product
obtained in Synthetic Example 1 was made, using an independent
multi-port-type specific surface area/pore distribution measuring
device (QUADRASORB SI, manufactured by Qantachrome).
[0105] The mesoporous carbon material in an amount of 0.067 g,
which was dried at 200.degree. C. for 2 hours as a pretreatment,
was utilized in measurement. As a result, it was found that the
specific surface area was 600 cm.sup.2/g and the average pore
diameter was 7 to 8 nm. (FIG. 1)
(SEM Observation of the Mesoporous Carbon Material Obtained in
Synthetic Example 1)
[0106] The mesoporous carbon material obtained in Synthetic Example
1, was coated with platinum, followed by observing with a field
emission scanning microscope (JSM-7000F, manufactured by JEOL), to
find pores having a diameter slightly smaller than 10 nm. (FIG.
2)
(TEM Observation of the Mesoporous Carbon Material Obtained in
Synthetic Example 1)
[0107] The mesoporous carbon material obtained in Synthetic Example
1, was observed by using a transmission electron microscope (H9000,
manufactured by HITACHI). As a result, it was found a portion where
pores having a diameter slightly smaller than 10 nm existed at
random, and a portion where the pores were formed in a hexagonal
structure and exactly aligned. (FIG. 3)
[0108] However, such a state was observed that the pores which
seemed to be arranged at random were arranged in a hexagonal form
at a certain angle in the course of changing the angle of the
sample stage from +58.degree. to -68.degree.. In other words, it is
assumed that all the pores of the mesoporous carbon material
obtained in Synthetic Example 1, had a hexagonal structure.
(Small-Angle Scattering XRD Measurement of the Mesoporous Carbon
Material Obtained in Synthetic Example 1)
[0109] Using a nano-scale X-ray structure evaluation device
(Nano-Viewer, manufactured by RIGAKU), the X-ray structure
diffraction of the mesoporous carbon material obtained in Synthetic
Example 1, was measured. As a result, a peak was observed at an
angle 2.theta. of 0.6.degree., and a second peak was slightly
observed at an angle from 1.degree. to 2.degree.. (FIG. 4)
[0110] It was thus confirmed that the mesopores had a regular pore
structure.
(Production of an Electrode Catalyst Support)
[0111] First, 500 mg of the mesoporous carbon material obtained in
Synthetic Example 1, was milled in a mortar. Then, 500 mg of the
mesoporous carbon material milled in the mortar, 19 mL of ethanol,
and 36 g of 1 mm.phi. zirconia balls were added in this order in a
zirconia container, and the resultant mixture was pulverized by a
planetary ball mill quartet (P-6, manufactured by FRITSCH), which
was rotated at 450 rpm, for 30 minutes, and that rotating operation
was repeated four times. After that, the beads were removed, and
the solvent was distilled off from the pulverized product, to
obtain a mesoporous carbon material as a powder.
(Production of an Electrode Catalyst Carrying 20% by Weight of Pt
on the Mesoporous Carbon Material)
[0112] To 100 mg of the powder mesoporous carbon material obtained
above, 3.3 mL of an aqueous solution including 59.4 mg of
tetraammineplatinate (II) nitrate (manufactured by Aldrich) was
added, and the mixture was stirred and dried at 20.degree. C. under
a nitrogen atmosphere for 1 hour. Then, the mixture was dried at
80.degree. C. under a nitrogen atmosphere for 18 hours. Thereto, 40
mL of dichloromethane (manufactured by Wako Pure Chemical
Industries Ltd.) was added, followed by stirring for 30 minutes.
The obtained mixture was subjected to a rotary evaporator
(manufactured by EYELA), to distill the solvent. The powdery sample
was taken out of the flask and poured into a quartz boat, followed
by heating under a nitrogen atmosphere by an infrared lamp heating
treatment device (ULVAC-RIKO, SSA-P610C). The sample was heated at
a rate of 1.degree. C./min from 20.degree. C. to 210.degree. C.,
kept at 210.degree. C. for 3 hours, heated at a rate of 1.degree.
C./min until the temperature was raised to 240.degree. C., and kept
at 240.degree. C. for 3 hours, to accomplish reduction to Pt
particles, thereby producing an electrode catalyst. Electrode
catalysts carrying 30% by weight of Pt and 40% by weight of Pt were
produced using 89.1 mg and 118.8 mg of tetraammineplatinate (II)
nitrate, respectively, in the same manner.
(Measurement of Pore Volumes Before and after the Deposition of Pt
on the Mesoporous Carbon Material)
[0113] The measurements of adsorption of nitrogen to the products
obtained in the above were made, using an independent
multi-port-type specific surface area/pore distribution measuring
device (QUADRASORB SI, manufactured by Qantachrome).
[0114] The mesoporous carbon materials were dried at 200.degree. C.
for 2 hours as a pretreatment. It was observed that the pore volume
was decreased with increase in the amount of Pt to be supported.
(FIG. 5)
(SEM Photograph of the Electrode Catalyst Carrying 30% by Weight of
Pt)
[0115] As a result of the observation using a field emission
scanning microscope (JSM-7000F, manufactured by JEOL), the presence
of platinum particles on the surface of the mesoporous carbon
material was not confirmed. In other words, it is assumed that the
reduction in pore volume was occurred not from the result that the
platinum particles (white particles) merely clogged the openings of
pores, but from the result that the platinum particles were
supported inside of pores. (FIG. 6)
(TEM Photograph of the Electrode Catalyst Carrying 30% by Weight of
Pt)
[0116] The sample was observed using a transmission electron
microscope (H9000, manufactured by HITACHI), and as a result, it
was found that aggregates of platinum particles (black) about 20 nm
in grain size locally existed, and also dispersion of platinum
particles (black) about 1.3 nm in grain size locally observed.
(FIG. 7, FIG. 8)
(Accurate Thermogravimetric Measurement of the Electrode Catalysts
Carrying 30% by Weight of Pt and 40% by Weight of Pt)
[0117] The actual amount (%) of the deposited platinum in the
electrode catalyst was measured using an accurate thermogravimetric
device (TGA-50, manufactured by SHIMADZU). The amounts of Pt in the
electrode catalysts carrying 30% by weight of Pt and 40% by weight
of Pt were 21.8% and 27.5%, respectively.
(Surface Area of Platinum of the Electrode Catalysts Carrying 40%
by Weight of Pt)
[0118] The surface area of platinum was calculated, based on the
amount of platinum supported which was found from the accurate
thermogravimetric measurement, and on the amount of hydrogen
adsorbed which was measured by a high-speed specific surface
area/pore distribution measuring device/micropore chemical
adsorbing system (ASAP2020C, manufactured by Micromeritics), to
find that the surface area of platinum was 42.7 m.sup.2/g Pt.
(Production of a Cathode of Example 1)
[0119] The surface of carbon rod (manufactured by Tokai Carbon) was
polished with a 0.3 .mu.m alumina paste for 10 minutes and with a
0.05 .mu.m alumina paste for 10 minutes. Then, the polished carbon
rod was subjected to ultrasonic cleaning with deionized water, and
then degreasing with ethanol. The catalyst carrying 10 mg of Pt
particles in pores of the mesoporous carbon material obtained
above, was dispersed in 10 mL of propanol (manufactured by Wako
Pure Chemical Industries Ltd.), by operating a ultrasonic
homogenizer (manufactured by SONICS) for 10 minutes. Then, 14 .mu.L
of the resultant dispersion was cast, on a 5 mm.phi. carbon rod,
followed by drying.
[0120] Further, a 5% Nafion solution (manufactured by Aldrich) was
diluted 100 times with ethanol (manufactured by Wako Pure Chemical
Industries Ltd.), and 9 .mu.L of the diluted solution was cast, to
the carbon rod, followed by drying, to produce a cathode of Example
1.
(Evaluation of an Oxygen-Reduction Reaction)
[0121] Using the cathodes produced in Example 1 and Reference
Example (a mesoporous carbon support carrying no Pt) as a working
electrode, a grassy carbon rod (manufactured by Tokai Carbon) as a
counter electrode, and a double junction silver/silver chloride
reference electrode (manufactured by ECO CHEMIE) as a reference
electrode, the oxygen-reduction characteristics were evaluated in
0.1 M perchloric acid solution (a product manufactured by Wako Pure
Chemical Industries Ltd. was diluted upon use) saturated with
oxygen, by a potentiostat/galvanostat (AUTOLAB, manufactured by ECO
CHEMIE). The results are shown in FIG. 9.
[0122] It can be seen that in the case of scanning at a rate of 50
mV/s between 1.0 V and -0.25 V, in the oxygen-reduction reaction,
the catalyst in Reference Example (only the mesoporous carbon
support) exhibited no activity, and the electrode catalyst
according to the present invention (the mesoporous material
carrying platinum) exhibited activity.
[0123] Further, the oxygen-reduction characteristics of the cathode
produced using only the catalyst carrying 30% by weight of Pt (no
Nafion) were evaluated. When this catalyst was scanned at a rate of
20 mV/s between 1.0 V and -0.25 V, the oxygen-reduction reactivity
was low, but the cathode electrode to which Nafion was added
exhibited a higher activity, and when the amount of Nafion was 4.5
.mu.L, the electrode catalyst exhibited the same performance as the
electrode catalyst obtained when the added amount of Nafion was 9
.mu.L. (FIG. 10)
[0124] That is, it is assumed that it was possible to form a
triple-phase interface in mesopores, by adding Nafion.
[0125] Further, a 5% Nafion solution (manufactured by Aldrich) was
diluted 100 times with ethanol, 2-propanol, and water/ethanol
(40/60 vol) (manufactured by Wako Pure Chemical Industries Ltd.),
respectively, upon use. In the oxygen-reduction characteristics of
the cathode produced using the catalyst carrying 30% by weight of
Pt, the dependency on a solvent for diluting Nafion was evaluated.
When scanning at a rate of 20 mV/s between 1.0 V and -0.25 V, the
system using Nafion diluted with water/ethanol had the lowest
activity, and the system using Nafion diluted with 2-propanol had
the highest activity. (FIG. 11)
[0126] This is assumed that since the inside of pores of the
mesoporous carbon material was hydrophobic, the use of a Nafion
solution diluted with a more hydrophobic solvent made it possible
to add Nafion in a larger amount in the pores, which accelerated
the formation of a triple-phase interface, and therefore the
oxygen-reduction reactivity was enhanced.
(Evaluation of Durability)
[0127] Using the cathode produced in Example 1 as the working
electrode, a grassy carbon rod (manufactured by Tokai Carbon) as
the counter electrode, and a double junction silver/silver chloride
reference electrode (manufactured by ECO CHEMIE) as the reference
electrode, the oxygen-reduction characteristics were evaluated in
0.1 M perchloric acid solution (a product manufactured by Wako Pure
Chemical Industries Ltd. was diluted upon use) saturated with
oxygen, by a potentiostat/galvanostat (AUTOLAB, manufactured by ECO
CHEMIE).
[0128] Even if the catalyst was scanned at a rate of 50 mV/s,
between 1.0 V and -0.25 V, under an argon atmosphere after this
scanning cycle was repeated 10,000 times under an oxygen
atmosphere, almost no change was observed in the hydrogen
adsorption wave of platinum. Further, the growth of platinum
particles was restricted.
Example 2
Production of a Mesoporous Carbon Membrane
[0129] Resorcinol (manufactured by Wako Pure Chemical Industries
Ltd.) in an amount of 0.41 g was dissolved in 1.09 g of deionized
water/1.44 g of ethanol (manufactured by Wako Pure Chemical
Industries Ltd.)/0.375 mL of 5M hydrochloric acid (manufactured by
Wako Pure Chemical Industries Ltd.). To the resultant solution,
0.236 g of Pluronic F-127 (manufactured by SIGMA) was added, to
dissolve the resultant mixture completely. Then, 0.30 g of triethyl
orthoacetate (manufactured by Wako Pure Chemical Industries Ltd.)
and 0.34 g of 37% formaldehyde manufactured by Wako Pure Chemical
Industries Ltd.) were added thereto, and the resultant mixture was
stirred at 30.degree. C. for 20 minutes. Then, 3 .mu.L of this
solution was cast, on a carbon rod (manufactured by Tokai Carbon),
which was then heated at 95.degree. C. for 6 hours in an air
blowing thermohygrostat (DMK300, manufactured by Yamato Scientific
Co., Ltd.), to polymerize resorcinol and formaldehyde.
[0130] The obtained membrane was heated over three days according
to a day-by-day condition in a nitrogen atmosphere, by using an
infrared lamp heating treatment device (ULVAC-RIKO, SSA-P610C).
[0131] First day: the temperature of the system was raised at a
rate of 1.degree. C. per minute from 20.degree. C. to 400.degree.
C. Then, the temperature was kept at 400.degree. C. for 3
hours.
[0132] Second day and third day: the temperature of the system was
raised at a rate of 2.degree. C. per minute from 20.degree. C. to
800.degree. C. Then, the temperature was kept at 800.degree. C. for
3 hours.
[0133] As a result, the target mesoporous carbon membrane was
obtained as a composite of mesoporous carbon membrane/carbon
rod.
(Production of a Pt Nano-Sized Colloid Solution)
[0134] First, 10 mL of a hexachloroplatinic acid solution (200 mg
hexachloroplatinic acid (manufactured by Aldrich)/10 mL ethylene
glycol (manufactured by Wako Pure Chemical Industries Ltd.)) was
added to 10 mL of 0.5 M NaOH (200 mg NaOH (manufactured by Wako
Pure Chemical Industries Ltd.)/10 mL ethylene glycol (manufactured
by Wako Pure Chemical Industries Ltd.)). Then, the resultant
mixture was refluxed at 160.degree. C. under a nitrogen atmosphere
for 3 hours, to obtain a colloid solution of platinum nano-sized
particles about 1 nm in grain size (3.76 mg/mL ethylene
glycol).
(Production of a Cathode of Example 2)
[0135] Hydrochloric acid (manufactured by Wako Pure Chemical
Industries Ltd.) was added to the platinum nano-sized
colloid/ethylene glycol solution, and the resultant solution was
adjusted to pH 4 or less, to precipitate platinum particles once.
Then, the precipitated platinum particles were re-dispersed in
ethanol (manufactured by Wako Pure Chemical Industries Ltd.). This
solution was cast, on a mesoporous carbon membrane in an amount of
10 .mu.L, followed by drying, and further 10 .mu.L of
dichloromethane was added, on the above membrane, followed by
drying, to give a cathode of Example 2.
(Evaluation of an Oxygen-Reduction Reaction)
[0136] Using the cathode produced in Example 2 as a working
electrode, a grassy carbon rod (manufactured by Tokai Carbon) as a
counter electrode, and a double junction silver/silver chloride
reference electrode (manufactured by ECO CHEMIE) as a reference
electrode, the oxygen-reduction characteristics were evaluated in
0.1 M perchloric acid solution (a product manufactured by Wako Pure
Chemical Industries Ltd. was diluted upon use) saturated with
oxygen, by a potentiostat/galvanostat (AUTOLAB, manufactured by ECO
CHEMIE). The results are shown in FIG. 12.
[0137] It is found that when the catalyst was scanned at a rate of
50 mV/s between 1.0 V and -0.25 V, the electrode catalyst according
to the present invention exhibited activity in the oxygen-reduction
reaction, also in the case of producing the cathode by the method
described in Example 2.
Comparative Example 1
Synthetic Example of a 20 wt % Pt Deposited Carbon Black
Catalyst
[0138] First, 300 mg of carbon black (Vulcan XC-72R, manufactured
by CABOT) was dispersed in 5 mL of an aqueous solution in which
214.5 mg of hexachloroplatinic (IV) acid hexahydrate (manufactured
by Wako Pure Chemical Industries Ltd.) was dissolved. The resultant
dispersion was reduced by adding 0.78 mL of 37% formaldehyde
(manufactured by Wako Pure Chemical Industries Ltd.). Then, 0.5 M
NaOH (manufactured by Wako Pure Chemical Industries Ltd.) was
added, dropwise, to the dispersion until the pH thereof was
increased to 14. After that, the resultant mixture was stirred at
normal temperature for 15 minutes, and then refluxed at 90.degree.
C. for 3 hours. Further, 2 to 3 mL of 1 M hydrochloric acid was
added thereto, followed by filtration. The thus-obtained platinum
deposited carbon black was thoroughly washed with deionized water,
and dried under vacuum at 100.degree. C. for 10 hours, to
synthesize a 20 wt % Pt deposited carbon black catalyst.
(Production of a Cathode)
[0139] This electrode catalyst was utilized, to produce a cathode
of Comparative Example 1 in the same manner as in Example 1.
(Evaluation of an Oxygen-Reduction Reaction)
[0140] Using the cathodes produced in Example 1 and Comparative
Example 1 as a working electrode, a grassy carbon rod (manufactured
by Tokai Carbon) as a counter electrode, and a double junction
silver/silver chloride reference electrode (manufactured by ECO
CHEMIE) as a reference electrode, the oxygen-reduction
characteristics were evaluated in 0.1 M perchloric acid solution (a
product manufactured by Wako Pure Chemical Industries Ltd. was
diluted upon use) saturated with oxygen, by a
potentiostat/galvanostat (AUTOLAB, manufactured by ECO CHEMIE). The
results are shown in FIG. 13.
[0141] It can be seen that, in the case of scanning at a rate of 50
mV/s between 1.0 V and -0.25 V, the electrode catalyst (20Pt-MC800)
according to the present invention was quite higher in the activity
than the electrode catalyst (20Pt--CB) of Comparative Example 1 in
the oxygen-reduction reaction.
Comparative Example 2
Production of a Cathode
[0142] A cathode of Comparative Example 2 was produced in the same
manner as in Example 1, except that 50-wt % Pt deposited carbon
black (TEC 10V50E, manufactured by Tanaka Kikinzoku) was used as
the electrode catalyst in the production of the cathode of Example
1.
(Electrochemical Evaluation Under an Argon Atmosphere)
[0143] Using the cathodes (14 .mu.g Pt/cm.sup.2) produced in
Example 1 and Comparative Example 2 as a working electrode, a
grassy carbon rod (manufactured by Tokai Carbon) as a counter
electrode, and a double junction silver/silver chloride reference
electrode (manufactured by ECO CHEMIE) as a reference electrode,
the electrochemical characteristics were evaluated in 0.1 M
perchloric acid solution (a product manufactured by Wako Pure
Chemical Industries Ltd. was diluted upon use) saturated with
oxygen, by a potentiostat/galvanostat (AUTOLAB, manufactured by ECO
CHEMIE). The results are shown in FIG. 14 (Example 1: 20-wt % Pt
deposited mesoporous carbon material) and FIG. 15 (Comparative
Example 2: 50-wt % Pt deposited carbon black).
[0144] In the case of scanning at a rate of 50 mV/s between 1.25 V
and -0.25 V, a clear difference in hydrogen adsorption wave was
observed between these two. The diameter of platinum particles of
the electrode catalyst according to the present invention is
assumed to be small.
(Evaluation of an Oxygen-Reduction Reaction)
[0145] Using the cathodes produced in Example 1 and Comparative
Example 2 as a working electrode, a grassy carbon rod (manufactured
by Tokai Carbon) as a counter electrode, and a double junction
silver/silver chloride reference electrode (manufactured by ECO
CHEMIE) as a reference electrode, the oxygen-reduction
characteristics were evaluated in 0.1 M perchloric acid solution (a
product manufactured by Wako Pure Chemical Industries Ltd. was
diluted upon use) saturated with oxygen, by a
potentiostat/galvanostat (AUTOLAB, manufactured by ECO CHEMIE).
[0146] The results are shown in FIG. 16 (Example 1: 20-wt % Pt
deposited mesoporous carbon material) and FIG. 17 (Comparative
Example 2: 50-wt % Pt deposited carbon black).
[0147] It can be found that in the case of scanning at a rate of 50
mV/s between 1.0 V and -0.25 V, the electrode catalyst according to
the present invention had a higher current value at a more negative
potential, than the electrode catalyst of Comparative Example 2, in
the oxygen-reduction reaction. It is also found that, with respect
to the electrode catalyst according to the present invention, the
oxygen reduction reaction activity is independent of the number of
rotations of the electrode.
Example 3
[0148] First, 5 mL of a dichloromethane (manufactured by Wako Pure
Chemical Industries Ltd.) solution including 90.5 mg of platinum
(II) acetylacetonate (manufactured by Aldrich) was added to 100 mg
of the powdery mesoporous carbon material which was obtained in
Synthetic 1, and the resultant mixture was dried under vacuum under
stirring. The powdery sample was taken out of the flask and poured
into a quartz boat, which was then heated under a nitrogen
atmosphere by an infrared lamp heating treatment device
(ULVAC-RIKO, SSA-P610C). The sample was heated at a rate of
1.degree. C./min from 20.degree. C. to 210.degree. C., kept at
210.degree. C. for 3 hours, further heated at a rate of 1.degree.
C./min until the temperature was raised to 240.degree. C., and kept
at 240.degree. C. for 3 hours, to accomplish reduction to Pt
particles, thereby producing a 30 wt % Pt-ac electrode
catalyst.
[0149] The measurement of adsorption of nitrogen to the product was
made, using an independent multi-port-type specific surface
area/pore distribution measuring device (QUADRASORB SI,
manufactured by Qantachrome). The product had been dried at
200.degree. C. for 2 hours, as a pretreatment. It was observed that
the volume of pores were decreased after Pt nano-sized particles
were carried. (FIG. 18)
[0150] As a result of the observation using a field emission
scanning microscope (JSM-7000F, manufactured by JEOL), the presence
of platinum particles (white) on the surface of the mesoporous
carbon material was confirmed. (FIG. 19)
[0151] The sample was observed using a transmission electron
microscope (H9000, manufactured by HITACHI), and, as a result, it
was found that platinum particles (black) having a grain size of
about 2 to about 3 nm were quite well dispersed in the mesoporous
carbon material. (FIG. 20, FIG. 21)
[0152] The actual amount (%) of the deposited platinum in the
electrode catalyst was measured using an accurate thermogravimetric
device (TGA-50, manufactured by SHIMADZU), which was 24.0%.
[0153] The surface area of platinum was calculated, based on the
amount of platinum supported which was found from the accurate
thermogravimetric measurement, and on the amount of hydrogen
adsorbed which was measured by a high-speed specific surface
area/pore distribution measuring device/micropore chemical
adsorbing system (ASAP2020C, manufactured by Micromeritics), to
find that the surface area of platinum was 87.0 m.sup.2/g Pt.
[0154] Using this electrode catalyst, a cathode of Example 3 was
produced in the same manner as in Example 1. Using the cathode, a
grassy carbon rod (manufactured by Tokai Carbon) as the counter
electrode, and a double junction silver/silver chloride reference
electrode (manufactured by ECO CHEMIE) as the reference electrode,
the oxygen-reduction characteristics were evaluated in 0.1 M
perchloric acid solution (a product manufactured by Wako Pure
Chemical Industries Ltd. was diluted upon use) saturated with
oxygen, by a potentiostat/galvanostat (AUTOLAB, manufactured by ECO
CHEMIE). The results are shown in FIG. 22.
[0155] In the case of scanning at a rate of 20 mV/s between 1.0 V
and -0.25 V, in an oxygen-reduction reaction, the electrode
catalyst (30 wt % Pt-ac electrode catalyst) of Example 3 was
enhanced in the oxygen-reduction activity, as compared to the
electrode catalyst (30 wt % Pt deposited electrode catalyst) of
Example 1.
Example 4
[0156] First, 3 mL of an acetone (manufactured by Wako Pure
Chemical Industries Ltd.) solution including 90.0 mg of
hexachloroplatinic acid hexahydrate (manufactured by Aldrich) was
slowly added, dropwise, to 100 mg of the powdery mesoporous carbon
material obtained in Synthetic Example 1, and the resultant mixture
was dried under stirring. The powdery sample was taken out of the
flask and poured into a quartz boat, which was then heated under a
hydrogen atmosphere, by an infrared lamp heating treatment device
(ULVAC-RIKO, SSA-P610C). The powdery sample was heated to
300.degree. C. from 20.degree. C. over 2 hours, to reduce the
sample to Pt particles, thereby producing a 30 wt % Pt--Cl
electrode catalyst.
[0157] As a result of the observation using a field emission
scanning microscope (JSM-7000F, manufactured by JEOL), the presence
of platinum particles (white) on the surface of the mesoporous
carbon material was confirmed. (FIG. 23)
[0158] The sample was observed using a transmission electron
microscope (H9000, manufactured by HITACHI), and as a result, it
was found that platinum particles were locally observed, and a
portion where platinum particles (black) having a grain size of
about 1.5 nm existed and a portion where platinum particles (black)
having a relatively large grain size about 4 to about 8 nm in grain
size existed were both observed. (FIG. 24 and FIG. 25)
[0159] The actual amount (%) of the deposited platinum in the
electrode catalyst was measured using an accurate thermogravimetric
device (TGA-50, manufactured by SHIMADZU), which was 27.0%.
[0160] Using this electrode catalyst, a cathode of Example 4 was
produced in the same manner as in Example 1. Using the cathode, a
grassy carbon rod (manufactured by Tokai Carbon) as the counter
electrode, and a double junction silver/silver chloride reference
electrode (manufactured by ECO CHEMIE) as the reference electrode,
the oxygen-reduction characteristics were evaluated in 0.1 M
perchloric acid solution (a product manufactured by Wako Pure
Chemical Industries Ltd. was diluted upon use) saturated with
oxygen, by a potentiostat/galvanostat (AUTOLAB, manufactured by ECO
CHEMIE). The results are shown in FIG. 26.
[0161] In the case of scanning at a rate of 20 mV/s between 1.0 V
and -0.25 V, in an oxygen-reduction reaction, the electrode
catalyst (30 wt % Pt--Cl electrode catalyst) of Example 4 was
drastically enhanced in the oxygen-reduction activity, as compared
to the electrode catalyst (30 wt % Pt deposited electrode catalyst)
of Example 1 and the electrode catalyst (30 wt % Pt-ac electrode
catalyst) of Example 3.
* * * * *